The present invention relates generally to power conversion systems and more particularly to a system for controlling the torque of an a.c. (alternating current) motor, by controlling the torque of the motor through the control of the motor current, to maximize the motor efficiency through control of the motor flux, and to provide for dynamic braking of the motor.
Historically, d.c. (direct current) motors have been used where operation over a wide speed range is desired. More recently, a.c. motors have been finding greater application in variable speed drive applications. This is due in large part because of the inherent ruggedness of an a.c. motor plus a lesser maintenance problem due to the lack of brushes which make a.c. motors desirable for certain applications.
There are, however, certain problems associated with the use of a.c. motors particularly when the motor is supplied with power from a variable frequency inverter such as a phase controlled thyristor inverter. This is primarily the result of the fact that the commutation delay of the motor current may be as great as 120 degrees. In any phase controlled converter situation, a certain amount of time is expected between the time of rendering conductive (firing) one thyristor in the converter until the current will transfer from a previously conducting to the newly fired thyristor. Normally, this overlap of time is from 10 to 30 electrical degrees. In the case of an inverter situation supplying an a.c. motor such as an induction motor at high motor speed, it may take as long as 120 electrical degrees between the firing of a new thyristor and the transfer of the current to the motor phase to which the thyristor is connected. As such, the use of open loop systems in which the thyristors are fired as a result of a preestablished schedule is not particularly applicable to this type of operation and instead a closed loop system in which the thyristors of the inverter are fired as a function of the existing angle between air gap flux and stator current (hereinafter referred to as "air gap power factor") represents a more desirable mode of operation.
In addition, because the a.c. current supplied to the a.c. motor from a current source inverter is not sinusoidal, the motor tends to exhibit torque pulsations which can become particularly objectionable at low speeds. At higher speeds, these pulsations are absorbed by motor inertia but at lower speeds, as the inverter approaches zero frequency, the frequency of the torque pulsations can become low enough to excite mechanical resonances in the structure or to provide what is generally referred to as cogging in the motor performance.